Not applicable.
The present invention relates to a method and arrangement for surface treatment of a paper and/or board web in a paper or board machine including a yankee cylinder followed by a calendering unit.
This kind of a machine is known from JP-A-05 331793. In the machine according to this JP-publication, the web""s dried side is dried in the Yankee cylinder and the calendering unit being formed by soft calender. A soft calender has a short nip time, therefore final results, f.ex. web glazing, are difficult to manipulate.
Calendering is a method by means of which the properties, in particular smoothness and gloss, of a web-like material, such as a paper web, are improved after a drying unit in a paper or board machine. In calendering the paper web is passed into a nip which is formed between rolls pressed against each other and in which the paper web is deformed by the action of temperature, moisture and nip pressure, in which connection the physical properties of the paper web can be affected by controlling the above-mentioned parameters and the time of action.
Extended-nip calendering has been found to be a good means of producing low-gloss paper grades by calendering. When higher gloss is required, the nip pressure of extended-nip calendering does not necessarily suffice to provide gloss.
With ever-increasing running speeds, calendering is becoming a bottleneck in the papermaking process, and satisfactory quality is not achieved by today""s machine calender units. Some of the drawbacks of the present papermaking process are also that the loss of bulk increases when gloss and smoothness are improved, and that in order to provide gloss and smoothness of sufficient quality, it is necessary to use webs with an abundance of coating and/or to use off-line calendering, in particular multi-nip supercalendering and/or soft calendering.
Machine calendering means here and hereafter calendering in a calender unit in which nips are formed between metal rolls. The width of the nip in a machine calender is typically very small depending on the width of the rolls and the thickness of the paper web to be calendered, wherefore the nip load is relatively high.
Supercalendering, which provides in off-line operation in practice the best result qualitywise, means above and hereafter calendering in a calender unit in which nips are formed between a metal or chilled thermoroll and a paper or polymer roll provided with a resilient surface, in which connection a nip of a substantial width is formed.
Soft calendering means above and hereafter calendering in a calender unit in which nips are formed in a manner similar to that of a supercalender between a metal or chilled roll and a roll having a resilient surface, in which connection a nip of a substantial width is formed. In soft calenders, each nip is formed between separate roll pairs, so that the nip load can be adjusted in each individual nip.
Shoe and/or extended-nip calendering means above and hereafter calendering in a calender unit in which a nip is formed between a roll provided with a flexible mantle, the mantle of said roll being made, for example, of polyurethane, and a press roll or shoe roll which has a rigid mantle and is provided with an inside loading shoe and which is made of metal, such as steel. One extended-nip concept marketed by the applicant is called OptiDwell(trademark), which includes two different extended-nip calenders:
OptiDwell Shoes(trademark) calender based on shoe press technology,
OptiDwell Belts(trademark) calender based on roll/belt technology.
A yankee cylinder represents a drying and glazing method known in the art for a long time, and the aim of the yankee cylinder is primarily to improve gloss but not to evaporate water. Yankee cylinders are employed mainly on paper and board machines which manufacture high-quality folding boxboards and envelope paper. On the yankee cylinder, the moisture of the web is about 65% at the most when it sticks to the surface of the cylinder and about 7-10% at the most when it is separated from the surface of the cylinder. The main problem with the yankee cylinder is, thus, its speed dependence. The evaporation capacity of the yankee cylinder is limited, so when the speed is increased, the surface of the web is no longer glazed to a satisfactory degree.